Such energy processes and systems have been known for many years. Major limitations of these systems have resulted from practical application of what seems to be a simple direct chemical process. Hazardous materials, efficiencies, system size, plugging and clogging, gas formation, "plating out" or precipitation of the materials, membrane diffusion limitations, cost of materials and cost of operation highlight the practical problems. Another limitation of such systems is the loss of power output as the system discharges.
Much work has been carried out in the development of air breathing fuel cells. A limitation is the requirement of an electrode able to sustain, support or catalyze the reaction at a rate great enough for a particular application. Other limitations of the known air electrodes are: high electrical resistance, short lifetimes, and the required hydrophilic and hydrophobic capabilities.
The known air electrodes are of porous construction with one side in contact with the air and another side in contact with an electrolyte. The electrolyte must be prevented from flooding the entire electrode. The air/oxygen must be allowed to contact the electrode and the electrolyte in a three way interface. It has been proposed to make one side of the electrode hydrophobic and the other side of the electrode hydrophilic in an attempt to maintain a delicate balance somewhere within the electrode. The reaction which is involved at the air electrode in an alkaline solution is: EQU 2e.sup.- + 1/2O.sub.2 +H.sub.2 O.fwdarw.2OH.sup.- Eq. 1
U.S. Pat. No. 3,634,140 discloses a fuel cell device utilizing air as the oxidant in which the flow through air electrode comprises two porous layers having different porosity, the layer located nearest the electrolyte side having the finer pores. The air atmosphere side and the electrolyte side are operated with a difference in pressure to prevent flooding of the pores of the air electrode.
U.S. Pat. No. 4,444,852 discloses a multiply laminar flow through air electrode which has an outer hydrophobic sheet adapted to contact the air and an inner hydrophilic electrolyte permeable active layer, the two outer layers being separated by one or more active layers comprising a mixture of active hydrophobic and hydrophilic agglomerates with catalyst disposed therein which increase in agglomerate size from the electrolyte permeable to the outer hydrophobic sheet.
GB-A-1364795 discloses a fuel cell in which a foaming agent is added to the electrolytes and the electrolytes agitated to form a foam containing the fuel and a foam containing the oxidant, the anode and the cathode being formed from smooth platinum and being positioned substantially only in the foam layers.
U.S. Pat. No. 4,328,287 discloses gas flow through electrodes for electrochemical cells having a porous flow through matrix with electrocatalysts supported on the surfaces of the matrix within the pores, the pores providing a through route for electrochemically oxidisable or reducible gas bubbles in a liquid electrolyte.
The zinc/air battery has been known for many years. This battery has a porous carbon +.sup.ve electrode, KOH paste electrolyte and a zinc -.sup.ve electrode. The original zinc/air battery was of U-shaped configuration and had the limitation of a slow discharge rate as the rate of air diffusing through the porous carbon electrode is proportional to the surface area of the carbon electrode in contact with the air. There is thus a limited mass transfer. Furthermore, this battery is essentially a primary battery and is not effectively electrically rechargeable because on attempting to recharge the battery zinc is redeposited unevenly at the zinc electrode, which limits the number of recharging cycles before the battery becomes inoperable. Furthermore, precipitates of zinc oxide and zinc carbonate are formed which cause contamination of the electrolytes and additional difficulties on attempting to recharge the system. An improved zinc/air battery employs a flat zinc plate as the -.sup.ve electrode and this enables more effective utilization of air/oxygen to be made by blowing air past the electrode face. There is thus a better mass transfer and hence a higher discharge rate than the U-shaped zinc/air battery, but the battery still suffers the same problems with regard to rechargeability.
Some attempts have also been made to replace the zinc in the zinc/air battery with other metals such as aluminium, iron, vanadium and magnesium. These devices have not, however, been satisfactorily electrically rechargeable.
U.S. Pat. No. 3,925,100 discloses an air cathode for use in metal/air cells such as zinc/air batteries which comprises a hydrophobic layer laminated to a hydrophilic layer. The air or oxygen is in contact with the hydrophobic layer through which it diffuses into the hydrophilic layer, but the electrolyte cannot pass through the hydrophobic layer.
It is an object of this invention to provide an electrochemical cell containing an improved air electrode for power generation and/or energy storage which is preferably electrically rechargeable and which provides economical power generation.
It is a further object of this invention to provide an electrochemical cell which contains an improved air electrode which is bipolar, which results in less costly cell arrays and greater volumetric energy densities.
It is a yet further object of this invention to provide an electrochemical cell with an improved air electrode with hydrophobic properties and which reduces problems associated with the loss of water from the prior art systems.
Another object of this invention is to provide an improved air electrode which provides substantially full power even at low states of charge of the system, i.e. the system power production stays constant over time maintaining substantially the same output until close to complete discharge.